As a semiconductor device is being miniaturized and highly integrated, a circuit pattern thereof is being miniaturized. As a circuit pattern is becoming miniaturized, in an optical lithography process for patterning a semiconductor device, required resolution is becoming high. As a result, NA of a projection optical system of an exposing device is becoming large. The wavelength of an exposing light source is becoming short.
The resolution (R) in the optical lithography is expressed by the following Rayleigh's formula.R=k1×{λ/(NA)}  (1)where λ represents the wavelength of exposure light; NA represents the numerical aperture of the exposing device; k1 represents a constant that depends on the condition and so forth of the optical lithography.
As the required resolution (R) is becoming high, since the k1 factor is becoming low, it becomes difficult to obtain the desired resolution.
In addition, as the performance and integration of a semiconductor device are becoming high in recent years, the required resolution is becoming high. As a result, an optical lithography in a region of which k1 is small, namely low-k1 lithography is performed.
In the low-k1 lithography, the mask production error of a photo mask largely affects the line width of a pattern (hereinafter referred to as critical dimension (CD)) of a transferred pattern transferred to a wafer.
Thus, in the low-k1 lithography, an inclined light is used for an exposing device. A phase shift mask is used as a photo mask. In addition, an OPC (Optical—Proximity Effect Correction) that bias-corrects the dimensions and shape of a mask and deforms the mask is used so as to correct the optical proximity effect.
In the OPC correction, as will be described later, a process model is extracted from a transferred result of a process model extracting mask. A mask corrected value is obtained in accordance with the process model. A corrected mask is produced in accordance with the obtained mask correction value.
As a factor that represents the emphasis rate of the influence of the pattern dimensional error of a photo mask (mask CD error) to the dimensional error of a transferred pattern transferred on a wafer (wafer CD error), an MEF (Mask Error Enhancement Factor) expressed by the following formula (2) is generally used.MEF=wafer CD error/(mask CD error/M)   (2)where M represents a reduction projection magnification of an exposing device. M is typically 5 or 4 for an exposing device used for producing a semiconductor device.
In the low-k1 lithography used in recent years, the MEF calculated by the formula (2) for a critical pattern may become 2 to 3. In other words, in the low-k1 lithograph, the dimensional error of a pattern of a photo mask largely impacts the dimensions of a transferred pattern transferred to a wafer. Thus, the influence of the dimensional error of a pattern of a photo mask against a transferred pattern is becoming large.
Next, with reference to FIG. 8, a conventional OPC correcting method for a mask will be described. FIG. 8 is a flow chart showing a procedure of a conventional OPC correcting method for a mask.
First of all, as shown in FIG. 8, at step S1, a mask for extracting a process model, namely a test mask, is produced. The process model is a function model of which a simulated result of a transferred pattern of a photo mask using a function model matches a measured result of a transferred pattern of a test mask. Test masks are produced so that test patterns having various shapes, dimensions, and pitches that represent a circuit pattern of a real device are placed on test masks.
Thereafter, the flow advances to a test pattern transferring step, step S2. At step S2, process conditions such as an exposing condition of the exposing device, a resist process condition, and an etching process condition are set. A test mask is transferred to a wafer under the process conditions that have been set. The transferred test mask is processed. The transferred pattern is formed on the wafer.
Thereafter, at step S3, the pattern dimensions of the transferred pattern formed on the wafer are measured by an SEM (Scanning Electron Microscope) or the like. Thereafter, at a process model extracting step, step S4, a process model is created in accordance with the measured values of the pattern dimensions.
Thereafter, at a corrected mask producing step, step S5, a corrected mask pattern that allows desired pattern dimensions and pattern shape to be obtained (extracted) after the test mask has been transferred and processed in accordance with the process model. A corrected mask is produced in accordance with the corrected mask pattern.
In the foregoing steps, a corrected mask that has been corrected by the OPC, namely a product mask, can be produced.
Thus, the OPC correction is performed by steps of production of test mask→transfer→measurement of dimensions→extraction of process model→obtainment of mask corrected values→production of corrected mask (product mask). The transferred pattern is measured with the corrected mask. As a result, the corrected mask is evaluated. Thus, the OPC correction for a mask is performed by complicated steps.
A conventional OPC correction for a mask is described in for example Japanese Patent Application Publication No. 2002-122977, pp 2-3.
In the low-k1 lithography used in recent years, the impact of the dimensional error of a pattern that takes place in production of a mask is becoming very large because a mask pattern should be more accurately transferred than before.
On the other hand, it is very difficult to produce a mask pattern of a photo mask in accordance with its design. The dimensions of a mask always contain a dimensional error of a pattern (tolerance). In particular, a problem to be solved is dependency of coarse/dense pattern or an error of line width depending on coarse/dense pattern.
As shown in FIG. 1, even if target line widths of mask patterns of photo masks are the same, the dimensional error of a pattern of dense lines is different from the dimensional error of a pattern of isolated lines. There is a tendency of which the dimensional error of a pattern of dense lines is larger than the dimensional error of a pattern of isolated lines.
For example, as reported with Proc. SPIE VOL. 4754 (2002) pp. 196-204, “Advanced pattern correction method for fabricating highly accurate reticles”, although the dimensional error of a pattern of a mask mainly results from a drawing error of a mask pattern and an etching error at an etching step performed after a patterning step and a developing step have been performed, dimension fluctuating coarse/dense characteristic, namely an error of line width depending on coarse/dense pattern is becoming considered.
An error due to drawing in production of a mask is corrected by EB (electron beam) exposure amount correction. An error due to etching of a substrate performed after a drawing step and a developing step is corrected by a pattern correction. However, it is difficult to completely control the error of line width depending on coarse/dense pattern.
As conventional photo mask specifications, there are a means dimensional tolerance (mean to target) and evenness of line width on a plane. However, the error of line width depending on coarse/dense pattern has not been mentioned in a roadmap such as ITRS (International Technology Roadmap for Semiconductors).
However, as will be described in the following, an error of line width depending on coarse/dense pattern of a mask largely affects a transferred CD error of a transferred pattern.
So far, at an optical lithography step, dimensional errors of patterns of photo masks have not been largely concerned. However, in the low-k1 lithography used nowadays, it is considered that two photo masks produced in accordance with the same specifications have transferred CDs of transferred patterns that largely differ.
This is because a small difference between two photo masks with respect to an error of line width depending on coarse/dense pattern is emphasized by a large MEF of the low-k1 lithography used in recent years.
A characteristic of a transferred CD in the case that a pattern pitch of a mask pattern is varied, namely a graph showing the relation between a pattern pitch and a transferred CD, is referred to as an OPE (Optical Proximity Effect) curve. This characteristic is one of basic data from which a mask corrected value is obtained.
Thus, an OPE curve as basic data with which a mask is corrected is affected by both an optical proximity effect and an error of line width depending on coarse/dense pattern.
As described above, each of a mask pattern of a test mask and a mask pattern of a corrected mask contains an error of line width depending on coarse/dense pattern of a mask. Thus, the difference between a test mask and a corrected mask with respect to an error of line width depending on coarse/dense pattern becomes important.
At the process model extracting step, step S4, assuming that a test mask has been produced in accordance with its design, a process model is produced. However, in reality, a mask pattern of a test mask contains an error of line width depending on coarse/dense pattern. Thus, since a process model that is produced contains a pattern dimension error, an error of line width depending on coarse/dense pattern of a test mask corresponding to an error of line width depending on coarse/dense pattern of a mask is propagated to a process model.
When a corrected mask having the same error depending on coarse/dense pattern as a test mask can be produced, the transferred CD error of the corrected mask can be decreased to almost zero. However, in reality, the difference between a test mask and a corrected mask with respect to an error of line width depending on coarse/dense pattern unavoidably takes place in the range of a production error. It is known that when a corrected mask is transferred, the difference between the masks is emphasized by the MEF as described above.
Thus, the difference between an error of line width depending on coarse/dense pattern of a test mask and an error of line width depending on coarse/dense pattern of a corrected mask, namely the difference between masks, affects the machining dimensions of the pattern of which the corrected mask is transferred. As a result, an unignorable error, namely, a mask corrected residual error, takes place in the transferred pattern.
As a result, it becomes difficult to obtain a transferred pattern having a high pattern dimensional control accuracy.
However, in the conventional mask correcting method using the OPC, the difference between masks is not specially considered. When a process model is produced by a test mask and a corrected photo mask corrected in accordance with the process model is produced, the influence of the difference between the test mask and the corrected photo mask with respect to the error of line width depending on coarse/dense pattern cannot be removed. The difference between the masks resides as an error element of the OPC corrected error. Thus, it becomes difficult to improve the correction accuracy.
Therefore, an object of the present invention is to provide a method for correcting a photo mask, namely a method for managing production of a mask that allows the difference between a test mask and a corrected mask with respect to an error of line width depending on coarse/dense pattern to be decreased when the photo masks are corrected by optical proximity effect correction.